Roller chain sprocket with added chordal pitch reduction

ABSTRACT

A sprocket is defined to include added chordal pitch reduction when in a new or “as-manufactured” condition. The sprocket chordal pitch P s ′ is purposefully reduced as compared to the as-built chain pitch P c  (or a theoretical maximum sprocket chordal pitch) a select amount referred to herein as “added chordal pitch reduction.” This added chordal pitch reduction is in addition to the inherent chordal pitch reduction that exists owing to manufacturing tolerances. A sprocket incorporating added chordal pitch reduction is highly desired in that it facilitates extension of the meshing time interval for an associated chain roller. The sprocket is formed to have a chordal pitch that is reduced relative to a maximum theoretical sprocket chordal pitch (or the chain link pitch) by an amount between about 0.2% and about 1% of the maximum theoretical sprocket chordal pitch.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.09/321,246 filed May 27, 1999, now U.S. Pat. No. 6.325.734, which is acontinuation of U.S. application Ser. No. 08/992,306 filed Dec. 17,1997, now U.S. Pat. No. 5,921,879, which claims priority from andbenefit of the filing date of U.S. provisional application serial No.60/032,379 filed Dec. 19, 1996.

BACKGROUND OF THE INVENTION

The present invention relates to the automotive timing chain art. Itfinds particular application in conjunction with a unidirectional rollerchain sprocket for use in automotive camshaft drive applications andwill be described with particular reference thereto. However, thepresent invention may also find application in conjunction with othertypes of chain drive systems and applications where reducing the noiselevels associated with chain drives is desired.

Roller chain sprockets for use in camshaft drives of automotive enginesare typically manufactured according to ISO (International Organizationfor Standardization) standard 606:1994(E). The ISO-606 standardspecifies requirements for short-pitch precision roller chains andassociated chain wheels or sprockets.

FIG. 1 illustrates a symmetrical tooth space form for an ISO-606compliant sprocket. The tooth space has a continuous fillet or rootradius R_(i) extending from one tooth flank (i.e., side) to the adjacenttooth flank as defined by the roller seating angle α. The flank radiusR_(f) is tangent to the roller seating radius R_(i) at the tangencypoint TP. A chain with a link pitch P has rollers of diameter D₁, incontact with the tooth spaces. The ISO sprocket has a chordal pitch alsoof length P, a root diameter D₂, and Z number of teeth. The pitch circlediameter PD, tip or outside diameter OD, and tooth angle A (equal to360°/Z) further define the ISO-606 compliant sprocket. The maximum andminimum roller seating angle α is defined as:

α_(max)=140°−(90°/Z) and α_(min)=120°−(90°/Z)

With reference to FIG. 2, an exemplary ISO-606 compliant roller chaindrive system 10 rotates in a clockwise direction as shown by arrow 11.The chain drive system 10 includes a drive sprocket 12, a drivensprocket 14 and a roller chain 16 having a number of rollers 18. Thesprockets 12, 14, and chain 16 each generally comply with the ISO-606standard.

The roller chain 16 engages and wraps about sprockets 12 and 14 and hastwo spans extending between the sprockets, slack strand 20 and tautstrand 22. The roller chain 16 is under tension as shown by arrows 24.The taut strand 22 may be guided from the driven sprocket 14 to thedrive sprocket 12 with a chain guide 26. A first roller 28 is shown atthe onset of meshing at a 12 o'clock position on the drive sprocket 12.A second roller 30 is adjacent to the first roller 28 and is the nextroller to mesh with the drive sprocket 12.

Chain drive systems have several components of undesirable noise. Amajor source of roller chain drive noise is the sound generated as aroller leaves the span and collides with the sprocket during meshing.The resultant impact noise is repeated with a frequency generally equalto that of the frequency of the chain meshing with the sprocket. Theloudness of the impact noise is a function of the impact energy (E_(A))that must be absorbed during the meshing process. The impact energyabsorbed is related to engine speed, chain mass, and the impact velocitybetween the chain and the sprocket at the onset of meshing. The impactvelocity is affected by the chain-sprocket engagement geometry, of whichan engaging flank pressure angle γ (FIG. 3) is a factor, where:$\begin{matrix}{{E_{A} = {\frac{wP}{2000}V_{A}^{2}}};} \\{{V_{A} = {\frac{\pi \quad {nP}}{30000}{\sin \left( {\frac{360}{Z} + \gamma} \right)}}};} \\{{\gamma = \frac{180 - A - \alpha}{2}};{and}}\end{matrix}$

E_(A)=Impact Energy [N·m]

V_(A)=Roller Impact Velocity [m/s]

γ=Engaging Flank Pressure Angle

n=Engine Speed [RPM]

w=Chain Mass [Kg/m]

Z=Number of Sprocket Teeth

A=Tooth Angle (360°/Z)

α=Roller Seating Angle

P=Chain Pitch (Chordal Pitch)

The impact energy (E_(A)) equation presumes the chain drive kinematicswill conform generally to a quasi-static analytical model and that theroller-sprocket driving contact will occur at a tangent point TP (FIG.3) of the flank and root radii as the sprocket collects a roller fromthe span.

As shown in FIG. 3, the pressure angle γ is defined as the angle betweena line A extending from the center of the engaging roller 28, when it iscontacting the engaging tooth flank at the tangency point TP, throughthe center of the flank radius R_(f), and a line B connecting thecenters of the fully seated roller 28, when it is seated on the rootdiameter D₂, and the center of the next meshing roller 30, as if it werealso seated on the root diameter D₂ in its engaging tooth space. Theroller seating angles α and pressure angles γ listed in FIG. 27 arecalculated from the equations defined above. It should be appreciatedthat γ is a minimum when α is a maximum. The exemplary 18-tooth, ISO-606compliant, sprocket 12 of FIG. 3 will have a pressure angle γ in therange of 12.5° to 22.5° as listed in the table of FIG. 27.

FIG. 3 also shows the engagement path (phantom rollers) and the drivingcontact position of roller 28 (solid) as the drive sprocket 12 rotatesin the direction of arrow 11. FIG. 3 depicts the theoretical case withchain roller 27 seated on root diameter D₂ of a maximum materialsprocket with both chain pitch and sprocket chordal pitch equal totheoretical pitch P. For this theoretical case, the noise occurring atthe onset of roller engagement has a radial component F_(IR) as a resultof roller 28 colliding with the root surface R_(i) and a tangentialcomponent F_(IT) generated as the same roller 28 collides with theengaging tooth flank at point TP as the roller moves into drivingcontact. It is believed that the radial impact occurs first, with thetangential impact following nearly simultaneously. Roller impactvelocity V_(A) is shown to act through, and is substantially normal to,engaging flank tangent point TP with roller 28 in driving contact atpoint TP.

The impact energy (E_(A)) equation accounts only for a tangential rollerimpact during meshing. The actual roller engagement, presumed to have atangential and radial impact (occurring in any order), would thereforeseem to be at variance with the impact energy (E_(A)) equation. Theapplication of this quasi-static model, which is beneficially used as adirectional tool, permits an analysis of those features that may bemodified to reduce the impact energy that must be absorbed during thetangential roller-sprocket collision at the onset of meshing. The radialcollision during meshing, and its effect on noise levels, can beevaluated apart from the impact energy (E_(A)) equation.

Under actual conditions as a result of feature dimensional tolerances,there will normally be a pitch mismatch between the chain and sprocket,with increased mismatch as the components wear in use. This pitchmismatch serves to move the point of meshing impact, with the radialcollision still occurring at the root surface R_(i) but not necessarilyat D₂. The tangential collision will normally be in the proximity ofpoint TP, but this contact could take place high up on the engaging sideof root radius R_(i) or even radially outward from point TP on theengaging flank radius R_(f) as a function of the actual chain-sprocketpitch mismatch.

Reducing the engaging flank pressure angle γ reduces the meshing noiselevels associated with roller chain drives, as predicted by the impactenergy (E_(A)) equation set forth above. It is feasible but notrecommended to reduce the pressure angle γ while maintaining asymmetrical tooth profile, which could be accomplished by simplyincreasing the roller seating angle α, effectively decreasing thepressure angle for both flanks. This profile as described requires thata worn chain would, as the roller travels around a sprocket wrap(discussed below), interface with a much steeper incline and the rollerswould necessarily ride higher up on the coast flank prior to leaving thewrap.

Another source of chain drive noise is the broadband mechanical noisegenerated in part by shaft torsional vibrations and slight dimensionalinaccuracies between the chain and the sprockets. Contributing to agreater extent to the broadband mechanical noise level is theintermittent or vibrating contact that occurs between the unloadedrollers and the sprocket teeth as the rollers travel around the sprocketwrap. In particular, ordinary chain drive system wear comprises sprockettooth face wear and chain wear. The chain wear is caused by bearing wearin the chain joints and can be characterized as pitch elongation. It isbelieved that a worn chain meshing with an ISO standard sprocket willhave only one roller in driving contact and loaded at a maximum loadingcondition.

With reference again to FIG. 2, driving contact at maximum loadingoccurs as a roller enters a drive sprocket wrap 32 at engagement.Engaging roller 28 is shown in driving contact and loaded at a maximumloading condition. The loading on roller 28 is primarily meshing impactloading and the chain tension loading. The next several rollers in thewrap 32 forward of roller 28 share in the chain tension loading, but ata progressively decreasing rate. The loading of roller 28 (and to alesser extent for the next several rollers in the wrap) serves tomaintain the roller in solid or hard contact with the sprocket rootsurface 34.

A roller 36 is the last roller in the drive sprocket wrap 32 prior toentering the slack strand 20. Roller 36 is also in hard contact withdrive sprocket 12, but at some point higher up (e.g., radiallyoutwardly) on the root surface 34. With the exception of rollers 28 and36, and the several rollers forward of roller 28 that share the chaintension loading, the remaining rollers in the drive sprocket wrap 32 arenot in hard contact with the sprocket root surface 34, and are thereforefree to vibrate against the sprocket root surfaces as they travel aroundthe wrap, thereby contributing to the generation of unwanted broadbandmechanical noise.

A roller 38 is the last roller in a sprocket wrap 40 of the drivensprocket 14 before entering the taut strand 22. The roller 38 is indriving contact with the sprocket 14. As with the roller 36 in the drivesprocket wrap 32, a roller 42 in the sprocket wrap 40 is in hard contactwith a root radius 44 of driven sprocket 14, but generally not at theroot diameter.

It is known that providing pitch line clearance (PLC) between sprocketteeth promotes hard contact between the chain rollers and sprocket inthe sprocket wrap, even as the roller chain wears. The amount of pitchline clearance added to the tooth space defines a length of a short arcthat is centered in the tooth space and forms a segment of the rootdiameter D₂. The root fillet radius R_(i) is tangent to the flank radiusR_(f) and the root diameter arc segment. The tooth profile is stillsymmetrical, but R_(i) is no longer a continuous fillet radius from oneflank radius to the adjacent flank radius. This has the effect ofreducing the broadband mechanical noise component of a chain drivesystem. However, adding pitch line clearance between sprocket teeth doesnot reduce chain drive noise caused by the roller-sprocket collision atimpact.

Chordal action, or chordal rise and fall, is another important factoraffecting the operating smoothness and noise levels of a chain drive,particularly at high speeds. Chordal action occurs as the chain entersthe sprocket from the free span during meshing and it can cause amovement of the free chain in a direction perpendicular to the chaintravel but in the same plane as the chain and sprockets. This chainmotion resulting from chordal action will contribute an objectionablenoise level component to the meshing noise levels, so it is thereforebeneficial to reduce chordal action inherent in a roller chain drive.

FIGS. 4a and 4 b illustrate the chordal action for an 18-tooth, ISO-606compliant, sprocket having a chordal pitch of 9.525 mm. Chordal rise 45may conventionally be defined as the displacement of the chaincenterline as the sprocket rotates through an angle A/2, where:

 Chordal rise =r _(p) −r _(c) =r _(p)[1−cos(180°/Z)]

where r_(c) is the chordal radius, or the distance from the sprocketcenter to a pitch chord of length P; r_(p) is the actual theoreticalpitch radius; and Z is the number of sprocket teeth.

It is known that a short pitch chain provides reduced chordal actioncompared to a longer pitch chain having a similar pitch radius. FIGS. 4aand 4 b show only the drive sprocket and assume a driven sprocket (notshown) also having 18-teeth and in phase with the drive sprocket shown.In other words, at T=0 (FIG. 4a), both sprockets will have a toothcenter at the 12 o'clock position. Accordingly, this chain drivearrangement under quasi-static conditions will have a top or taut strandthat will move up and down in a uniform manner a distance equal to thatof the chordal rise. At T=0, a roller 46 is at the onset of meshing,with chordal pitch P horizontal and in line with taut strand 22. At T=0+(A/2), (FIG. 4b), roller 46 has moved to the 12 o'clock position.

For many chain drives, the drive and driven sprockets will be ofdifferent sizes and will not necessarily be in phase. The chain guide 26(FIG. 2) has the primary purpose to control chain strand vibration inthe taut span. The geometry of the guide-chain interface also definesthe length of free span chain over which chordal rise and fall isallowed to operate. FIG. 5 is an enlarged view of FIG. 2 showing thefirst roller 28 at the onset of engagement and the second roller 30 asthe next roller about to mesh with sprocket 12. In this example, thechain guide 26 controls and guides the engaging portion of the tautstrand 22 except for five (5) unsupported or “free” link pitchesextending between the chain guide 26 and the engaging roller 28. Thislength of unsupported link pitches for the engaging portion of tautstrand 22 in this example is horizontal when roller 28 is at the 12o'clock position.

With reference to FIGS. 6 and 7, the drive sprocket 12 is rotated in aclockwise direction to advance roller 28 to a new angular position(A/2)+ω, where ω is the added rotation angle as determined by aquasi-static engagement geometry with roller 28 being fully seated androller 30 is at the instant of sprocket engagement. As shown in FIG. 6,roller 28 is considered to be seated and in hard contact with the rootsurface at D₂ at the onset of meshing of roller 30, and a straight lineis assumed for the chain span from roller 28 to a chain pin center 48,about which the unsupported or “free” span from pin 48 to engagingroller 30 is considered to rotate.

As a result of the chordal action, the engaging free span is no longerhorizontal to satisfy the roller engaging geometry. This is in contrastto the chain drive as described in FIG. 4a in which chordal actioncauses the taut strand to move uniformly, but in a horizontal pathbecause the drive and driven sprockets have the same number of teeth andthe sprocket teeth are in phase. It should be appreciated that thestraight line assumption is valid only in a quasi-static model. Theamount of movement or deviation from the straight line assumption willbe a function of the drive dynamics, the chain control devices, and thechain drive and sprocket geometry. The location and chain-interfacingcontour of the chain guide 26 will determine the number of free spanpitches for which chain motion will take place as a result of thechordal rise and fall during the roller meshing process.

As best seen in FIG. 7, assuming that rollers 28 and 30 are in hardcontact with the sprocket root surfaces at D₂, the chordal rise is theperpendicular displacement of the center of roller 30 (located on thepitch diameter PD) from the taut span 22 path as it moves from itsinitial meshing position shown through the 12 o'clock position.

Accordingly, it is desirable to develop a new and improved roller chaindrive system which meets the above-stated needs and overcomes theforegoing disadvantages and others while providing better and moreadvantageous results.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, aunidirectional roller chain drive system includes an as-manufactureddrive sprocket including a plurality of teeth, an as-manufactured drivensprocket including a plurality of teeth, and an as-built roller chainengaged with said drive sprocket and said driven sprocket. The rollerchain defines a chain link pitch (P_(c)). The drive sprocket and/or thedriven sprocket define(s) a sprocket chordal pitch (P_(s)′) that is lessthan the chain link pitch (P_(c)) by more than about 0.2% of the chainlink pitch (P_(c)).

In accordance with a more limited aspect of the present invention, thedrive sprocket and/or the driven sprocket define(s) a sprocket chordalpitch (P_(s)′) that is less than the chain link pitch (P_(c)) by morethan about 0.2% of the chain link pitch (P_(c)) but no more than about1% of the chain link pitch (P_(c)).

In accordance with another aspect of the present invention, anas-manufactured sprocket is provided that is adapted for use inconjunction with an associated as-built roller chain including aplurality of rollers and defining a chain pitch P_(c). The sprocketincludes a plurality of sprocket teeth projecting outwardly therefromand defining a plurality of tooth spaces located respectively betweensuccessive teeth. Each of the tooth spaces is adapted to receive aroller of the associated roller chain. The sprocket defines a sprocketchordal pitch (P_(s)′) that is reduced relative to the chain pitch P_(c)of the associated roller chain by more than about 0.2% of the chainpitch P_(c).

In accordance with a more limited aspect of the present invention, thesprocket chordal pitch (P_(s)′) is reduced relative to the chain linkpitch (P_(c)) by about 0.2% to about 1% of the chain link pitch.

In accordance with another aspect of the present invention, a method ofextending the meshing interval for an as-built roller chain and anas-manufactured sprocket is provided. The method includes defining thesprocket to have a sprocket chordal pitch (P_(s)′) that is reducedrelative to a chain link pitch (P_(c)) of the roller chain by more thanabout 0.2% of the chain link pitch (P_(c)).

In accordance with a more limited aspect of the present invention, thesprocket chordal pitch (P_(s)′) is reduced relative to the chain linkpitch (P_(c)) by more than about 0.2% of the chain link pitch (P_(c))and less than or equal about 1% of the chain link pitch (P_(c)).

A main advantage of the present invention resides in the provision of asprocket exhibiting improved noise characteristics when in use with anassociated roller chain of a roller chain drive system.

Another advantage of the present invention is found in the provision ofa sprocket wherein the meshing interval for the rollers of an associatedroller chain is lengthened to reduce noise during use.

A still further advantage of the present invention resides in theprovision of a sprocket and a roller chain drive system incorporatingthe sprocket wherein added chordal pitch reduction lengthens the meshingtime interval and, thus, serves to provide a more gradual load transferfrom a seated roller to the engaging roller at the onset of meshing,thereby spreading the meshing impact over a longer time period whichhelps to minimize the noise associated with roller impact.

Further advantages of the present invention will become apparent tothose of ordinary skill in the art upon reading and understanding thefollowing detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating a preferred embodiment and are notto be construed as limiting the invention.

FIG. 1 illustrates a symmetrical tooth space form for a ISO-606compliant roller chain sprocket;

FIG. 2 is an exemplary roller chain drive system having a ISO-606compliant drive sprocket, driven sprocket, and roller chain;

FIG. 3 shows an engagement path (phantom) and a roller (solid) in adriving position as an ISO-606 compliant drive sprocket rotates in aclockwise direction;

FIG. 4a shows a roller at the onset of meshing with an exemplary18-tooth sprocket;

FIG. 4b shows the drive sprocket of FIG. 4a rotated in a clockwisedirection until the roller is at a 12 o'clock position;

FIG. 5 is an enlarged view of the drive sprocket of FIG. 2 with a rollerfully seated in a tooth space and a second roller about to mesh with thedrive sprocket;

FIG. 6 shows the drive sprocket of FIG. 5 rotated in a clockwisedirection until the second roller initially contacts the drive sprocket;

FIG. 7 is an enlarged view of FIG. 6 showing that the second rollerinitially contacts a root surface (i.e., radial impact) of the drivesprocket, under theoretical conditions;

FIG. 8 illustrates a roller chain drive system having a roller chaindrive sprocket and driven sprocket which incorporate the features of thepresent invention therein;

FIG. 9 illustrates the roller chain drive sprocket of FIG. 8 with anasymmetrical tooth space form in accordance with one embodiment of thepresent invention;

FIG. 10 is an enlarged view of the asymmetrical tooth space form of FIG.9 showing a roller in two-point contact with the sprocket;

FIG. 11 shows an engagement path (phantom) and the instant of full mesh(solid) of a roller as the drive sprocket of FIG. 8 rotates in aclockwise direction;

FIG. 12 is an enlarged view of the drive sprocket of FIG. 8 with a firstroller fully seated in a tooth space and a second roller as the nextroller to be collected from the taut span of the roller chain;

FIG. 13 shows the drive sprocket of FIG. 12 rotated in a clockwisedirection until the second roller initially contacts the drive sprocket;

FIG. 14 is an enlarged view of FIG. 13 showing the first roller intwo-point contact and second roller at initial tangential contact withthe drive sprocket;

FIG. 14a illustrates the progression of the first and second rollers asthe drive sprocket of FIG. 14 is rotated in a clockwise direction;

FIG. 14b is an enlarged view of the drive sprocket of FIG. 14 rotated ina clockwise direction to advance the second roller to the instant offull mesh at a 12 o'clock position;

FIG. 15 illustrates a roller chain drive sprocket with an asymmetricaltooth space form in accordance with another embodiment of the presentinvention;

FIG. 16 is an enlarged partial view of FIG. 8, showing the contactprogression as the rollers travel around the drive sprocket wrap;

FIG. 17 is an enlarged view of a roller exiting a sprocket wrap of thesprocket of FIG. 16;

FIG. 18 illustrates a roller chain sprocket with an asymmetrical toothspace form in accordance with another embodiment of the presentinvention;

FIG. 19 illustrates a roller chain sprocket with an asymmetrical toothspace form in accordance with a further embodiment of the presentinvention;

FIG. 20 is a front view of an exemplary random-engagement roller chainsprocket which incorporates the features of the present inventiontherein;

FIG. 21 is an enlarged view of the sprocket of FIG. 20 showing anasymmetrical tooth space form which incorporates engaging flank reliefand tooth space clearance in accordance with the present invention;

FIG. 21a is an enlarged view of the sprocket of FIG. 21 showing aninclined root surface thereof which provides flank relief and toothspace clearance;

FIG. 22 is another embodiment of the inclined root surface of FIG. 21awhich only provides flank relief;

FIG. 23 illustrates the asymmetrical tooth space form of FIG. 9 overlaidwith the asymmetrical tooth space form of FIG. 21;

FIG. 24 illustrates the meshing progression of a first roller and themeshing of a second adjacent roller with the sprocket of FIG. 20 as thesprocket is rotated in a clockwise direction;

FIG. 25 illustrates the sprocket of FIG. 20 with a first roller intwo-point contact, a second roller at initial tangential contact, and athird roller as the next roller to engage with the drive sprocket;

FIG. 26 illustrates the sprocket of FIG. 25 rotated in a clockwisedirection until the instant of initial engagement of the third roller ata root surface of the sprocket;

FIG. 27 is a table listing roller seating angles α and pressure angles γfor a number of different ISO-606 compliant sprocket sizes;

FIG. 28 is a table listing the maximum Beta (β) angles and thecorresponding minimum pressure angles (γ) for three differentasymmetrical tooth space profiles (1-3) of varying sprocket sizes; and,

FIGS. 29-31 are included to illustrate a sprocket incorporating addedchordal pitch reduction (CPR) in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference now to FIG. 8, a roller chain drive system 110 includes adrive sprocket 112 and a driven sprocket 114 which incorporate thefeatures of the present invention therein. The roller chain drive system110 further includes a roller chain 116 having a number of rollers 118which engage and wrap about sprockets 112, 114. The roller chain movesaround the sprockets 112, 114, both of which rotate in a clockwisedirection as shown by arrow 11.

The roller chain 116 has two spans extending between the sprockets,slack strand 120 and taut strand 122. The roller chain 116 is undertension as shown by arrows 124. A central portion of the taut strand 122may be guided from the driven sprocket 114 to the drive sprocket 112with a chain guide 126. A first roller 128 is shown fully seated at a 12o'clock position on the drive sprocket 112. A second roller 130 isadjacent to the first roller 128 and is about to mesh with the drivesprocket 112.

To facilitate a description of the asymmetrical tooth profiles of thepresent invention, reference will be made only to the drive sprocket112. However, the asymmetrical tooth profiles of the present inventioncan be equally applied to the driven sprocket 114, as well as to othertypes of sprockets such as idler sprockets and sprockets associated withcounter rotating balance shafts.

Referring now to FIGS. 9 and 10, the sprocket 112 includes a first tooth132 having an engaging flank 134, and a second tooth 136 having a coastor disengaging flank 138. The engaging flank 134 and coast flank 138cooperate to define a tooth space 140 having a root surface 141. Thetooth space 140 receives the engaging roller 128 (shown in phantom). Theengaging roller 128 has a diameter D₁, and is shown fully seated intwo-point contact within the tooth space 140 as described further below.More particularly, the engaging roller 128, when fully seated in thetooth space, contacts two lines B and C that extend axially along eachsprocket tooth surface or face (i.e., in a direction orthogonal to theplane of the drawings). However, to facilitate a description thereof,the lines A, B, and C are hereafter shown and referred to as contactpoints within the tooth space.

The engaging flank 134 has a radius R_(f) which is tangent to a radiallyouter end of a flank flat 144. The asymmetrical engaging flank radiusR_(f) is smaller than the R_(fISO) radius specified by the ISO-606standard. However, the magnitude of the asymmetrical engaging flankradius R_(f) should be as large as possible while still satisfying theroller meshing (engaging) and disengaging geometry. The location of theflank flat 144 is defined by an angle β, with the flat orientation beingnormal or perpendicular to a line that passes through Point B and thecenter of roller 128 when the roller is contacting the sprocket atPoints B and C. The length of the flank flat, which extends radiallyoutward from Point B, affects a time delay between an initial tangentialimpact between sprocket 112 and roller 128 at a first contact Point Aalong the flank flat 144, and a subsequent radial impact at Point C. Itis believed that the roller stays substantially in contact with theflank flat from its initial tangential contact at Point A until theroller moves to a fully engaged two-point contact position at Points Band C. The pressure angle γ, the amount of pitch mismatch between thechain and the sprocket, and the length of the flank flat can be variedto achieve a desired initial roller contact Point A at the onset ofroller-sprocket meshing.

It should be appreciated that flank (tangential) contact always occursfirst, with radial contact then occurring always at Point C regardlessof chain pitch length. In contrast, with known tooth space forms (e.g.,ISO-606 compliant and asymmetrical) incorporating single-point contact(e.g. single line contact), an engaging roller must move to a drivingposition after making radial contact. The pressure angles γ thereforeassume that the engaging roller will contact at the flank radius/rootradius tangent point. Thus, the meshing contact location of the knownsingle point/line tooth space forms is pitch “sensitive” to determinewhere the radial impact as well as driving (tangential) impact willoccur.

The engaging flank roller seating angle β (FIG. 9) and a disengagingflank roller seating angle β′ replace the ISO-606 roller seating angle α(ISO profile shown in phantom). As illustrated in FIG. 9, β is the angledefined by the line 152 passing through the center of the roller 128 andthe sprocket center to a second line which also passes through thecenter of roller 128 and Point B. Also shown in FIG. 9, β′ is the angledefined by a line passing through the arc center of R_(i)′ and thesprocket center to a second line which also passes through the arccenter of R_(i)′ and the arc center of R_(f)′. The pressure angle γ is afunction of the engaging flank roller seating angle β. That is, as βincreases, γ decreases. A minimum asymmetrical pressure angle can bedetermined from the following equation, where:

γ_(min)=β_(max)−(α_(max)/2+γ_(ISO min))

Therefore, an asymmetrical pressure angle γ_(min)=0 whenβ_(max)=(α_(max)/2+γ_(ISO min)) as illustrated in the table of FIG. 28.FIG. 28 lists the maximum Beta (β) angles and the corresponding minimumpressure angles (γ) for several sprocket sizes of three differentasymmetrical tooth space profiles (1-3). It should be appreciated thatreducing the engaging flank pressure angle γ reduces the tangentialimpact force component F_(IA) (FIG. 14) and thus the tangential impactnoise contribution to the overall noise level at the onset ofengagement.

That is, the impact force F_(IA) is a function of the impact velocitywhich in turn is related to pressure angle γ. As pressure angle γ isreduced, it provides a corresponding reduction in the impact velocitybetween the chain and the sprocket at the onset of meshing. A minimumpressure angle γ also facilitates a greater separation or distancebetween tangential contact points A and B to further increase ormaximize the meshing interval. In the preferred embodiment, the engagingflank pressure angle γ is in the range of about −2.0° to about +5° tooptimize the staged impact between the roller and the sprocket.

In the embodiment being described, roller seating angle β is greaterthan ISO α_(max)/2 at a maximum material condition and β can be adjusteduntil a desired engaging flank pressure angle γ is achieved. Forinstance, the roller seating angle β of FIG. 9 provides a pressure angleγ that is less than zero, or a negative value. The negative pressureangle γ is best seen in FIG. 11, as contrasted with the ISO-606compliant tooth profile of FIG. 3 with a positive pressure angle γ. Asshown in FIG. 11, the asymmetrical profile pressure angle γ is definedas the angle between a line A extending from the center of the fullyengaged roller 128, when it is contacting the engaging tooth flank atpoints B and C, through point B, and a line B connecting the centers ofthe fully seated roller 128, and the center of the next meshing roller130 as if it were also two-point seated at full mesh in its engagingtooth space. It is believed that a small negative pressure angle for thetheoretical chain/sprocket interface beneficially provides a pressureangle γ closer to zero (0) for a “nominal” system or for a system withwear. However, the engaging flank roller seating angle β may bebeneficially adjusted so as to provide any engaging flank pressure angleγ having a value less than the minimum ISO-606 pressure angle.

Referring again to FIGS. 9 and 10, a first root radius R_(i) is tangentto a radially inner end of the flank flat 144, and tangent to a radiallyouter end of an inclined root surface 146. As best seen in FIG. 10, amaximum root radius R_(i) must be equal to, or less than, a minimumroller radius 0.5D₁ to facilitate the fully engaged two-point/linecontact at Points B and C. Accordingly, this will define a smallclearance 148 (FIG. 10) between the engaging flank 134 at root radiusR_(i) and the roller 128 at full mesh (i.e. two-point/line contact). Theflank flat 144 and the inclined root surface 146 necessarily extendinside Points B and C respectively to facilitate the two-point/lineroller contact at full engagement for all dimensional toleranceconditions of the roller 128 diameter D₁ and the root radius R_(i). Asecond root radius R_(i)′ is tangent to a radially inner end of theinclined root surface 146 at line 150. The disengaging flank radiusR_(f)′ is tangent to R_(i)′ at a point defined by disengaging flankroller seating angle β′. The radius R_(f)′ can have a value in theISO-606 range.

The inclined root surface 146 is a flat surface having a finite lengthwhich defines a tooth space clearance (TSC). The tooth space clearancecompensates for chain pitch elongation or chain wear by accommodating aspecified degree of chain pitch elongation ΔP. In other words, the toothspace clearance TSC enables rollers of a worn chain to be maintained inhard contact with the inclined root surface of the sprocket teeth. Inaddition, the inclined root surface 146 facilitates reducing the radialreaction force thereby reducing the roller radial impact noisecontribution to the overall noise level.

The inclined root surface 146 may be inclined at any angle φ necessaryto satisfy a specific chain drive geometry and chain pitch elongation.As shown in FIG. 9, the inclined root surface angle φ is measured from aline 152 passing through the center of roller 128 and the sprocketcenter to a second line 154 which also passes through the center ofroller 128 and Point C. The inclined root surface 146 is normal to theline 154, and the inclined root surface extends radially inward to line150 where it is tangent to R_(i)′. In the embodiment being described,the inclined root surface angle φ is preferably in the range of about20° to about 35°.

FIG. 12 is an enlarged view of FIG. 8 showing the first roller 128 atfull engagement in two-point/line contact across the thickness or widthof the sprocket tooth profile, and the second roller 130 as the nextroller about to mesh with sprocket 112. As with the ISO-606 compliantdrive system 10, the chain guide 126 controls and guides a centralportion of the taut strand 122 except for five unsupported link pitchesextending between the chain guide 126 and the engaging roller 128 (andexcept for the unsupported link pitches extending between the drivensprocket and the chain guide). The taut strand 122 is horizontal whenroller 128 is at the 12 o'clock position.

FIG. 13 shows the drive sprocket 112 rotated in a clockwise direction(A/2)+ω, as determined by the instant of sprocket engagement by roller130. A straight line is assumed for the chain span from roller 128 to achain pin center 156, about which the unsupported span from pin center156 to engaging roller 130 is considered to rotate. It should beappreciated that the straight line assumption is valid only in aquasi-static model. The amount of movement (or deviation from thestraight line assumption) previously mentioned will be a function of thedrive dynamics as well as the drive and sprocket geometry.

The sprocket contact at the onset of mesh for roller 130 occurs earlierthan for the ISO-606 counterpart, thereby reducing the amount of chordalrise and, just as importantly, allows the initial contact tobeneficially occur at a desired pressure angle γ on the engaging flankat Point A. Furthermore, the radial sprocket contact for roller 130,with its contribution to the overall noise level, does not occur untilthe sprocket rotation places roller 130 at the 12 o'clock position. Thisis referred to as lengthening the meshing interval.

FIG. 14, an enlarged view of FIG. 13, more clearly shows the onset ofmeshing for roller 130. Just prior to the onset of mesh, roller 128 isassumed to carry the entire taut strand load F_(TB)+F_(φ), which load isshown as force vector arrows. Actually, the arrows represent reactionforces to the taut strand chain force. At the instant of mesh for roller130, a tangential impact occurs as shown by impact force vector F_(IA).The tangential impact is not the same as the taut strand chain loading.In particular, impact loading or impact force is related to the impactvelocity V_(A). It is known that impact occurs during a collisionbetween two bodies, resulting in relatively large forces over acomparatively short interval of time. A radial impact force vectorF_(IC) is shown only as an outline in that the radial impact does notoccur until the sprocket has rotated sufficiently to place roller 130 ata 12 o'clock position.

FIG. 14a shows the same roller positions (solid) for rollers 128 and 130as shown in FIG. 14, but in addition, shows the roller positions (inphantom) relative to the sprocket profile once roller 130 reaches itstwo-point/line mesh at the 12 o'clock position. As a result of the pitchmismatch between the chain and sprocket, roller 128 must move to a newposition. In particular, as roller 130 moves from initial contact tofull mesh, roller 128 progresses forward in its tooth space. Smallclearances in the chain joints, however, reduce the amount of forwardprogression required for roller 128. Also occurring at the onset ofmeshing is the beginning of the taut strand load transfer from roller128 to roller 130.

The asymmetrical profile provides for the previously described “staged”meshing that exhibits a lengthened meshing interval. In particular,referring again to FIG. 14, the Point A tangential contact occurs at theonset of mesh, with its related impact force F_(IA). The roller 130 isbelieved to stay substantially in hard contact with the engaging flank134 as the sprocket rotation moves the roller into full mesh with itsresulting radial contact at Point C.

FIG. 14b is an enlarged view of FIG. 14, except that sprocket 112 hasbeen rotated to advance roller 130 to the instant of full mesh at the 12o'clock position. At this instant of full mesh, the radial impact forceF_(IC) occurs and the taut strand load transfer is considered to becomplete. At the instant of the radial collision by roller 130 at PointC, with its resultant radial impact force F_(IC), the tangential impactforce of F_(IA) has already occurred and is no longer a factor. The timedelay (“staged” engagement) between the tangential and radialroller-sprocket collisions effectively spreads the impact energy over agreater time interval, thereby reducing its contribution to thegenerated noise level at mesh frequency. Additionally, it is believedthat the present asymmetrical sprocket tooth profile beneficiallypermits a more gradual taut strand load transfer from a fully engagedroller 128 to a meshing roller 130 as the meshing roller 130 moves fromits Point A initial mesh to its full two-point mesh position.

Referring again to FIG. 14, the chordal rise (and fall) with the presentasymmetrical profile is the perpendicular displacement of the center ofroller 130 from the taut strand 122 path as it moves from its initialmeshing contact Point A to the mesh position presently occupied byroller 128. It is believed that roller 130 will stay in hard contactwith the engaging flank 134 as the roller moves from initial tangentialcontact to full mesh, and accordingly, the chordal rise is reduced asthe distance between Points A and B is increased. As shown in FIG. 14,chain pitch P_(C) is beneficially greater than sprocket 112 chordalpitch P_(S).

Referring now to FIG. 15, the length of the inclined root surface 146(FIG. 10) may be reduced to zero (0), thereby eliminating the inclinedroot surface 146 and permitting root radius R_(i)′ to be tangent to theroot surface and the roller 128 at Point C. That is, R_(i)′ is tangentto a short flat at Point C, and the flat is tangent to R_(i). If theinclined root surface 146 is eliminated, the engaging flank pressureangle γ would generally be in the range of some positive value to zero,but normally not less than zero. The reason is that a negative γrequires chordal pitch reduction so that the roller can exit thesprocket wrap 60 (FIG. 16) without interfering with R_(f).

FIG. 16 shows the roller contact to the sprocket 112 profile for all therollers in the wrap 60. Roller 128 is in full two-point mesh as shown.Line 160 shows the contact point for each of the rollers, as well as thecontact progression as the rollers travel around the wrap. The inherentpitch mismatch between the sprocket and roller chain causes the rollersto climb up the coast side flank as the rollers progress around thesprocket wrap. With the addition of appreciable chordal pitch reduction,the extent to which the rollers climb up the coast side flank isincreased.

It is important to note that chordal pitch reduction is required whenthe pressure angle γ has a negative value. Otherwise, as is evident froman examination of FIGS. 16 and 17, roller 162 would interfere with theengaging flank (with a maximum material sprocket and a theoretical pitch[shortest] chain) as it exits the wrap 60 back into the span. That is,chordal pitch reduction permits the roller 162 to exit the wrap 60 witha clearance 163 at the engaging flank. Also, the reduced chordal pitchserves to increase the meshing interval (staged meshing) as previouslymentioned. FIG. 16, showing the roller contact progression in the wrap60, serves also to show why the shallow β′ angle and tooth spaceclearance TSC helps maintain “hard” roller-sprocket contact for therollers in the wrap.

In addition, the disengaging flank roller seating angle β′ (FIG. 9) maybe adjusted to have a maximum value which is equal to α_(min)/2 or evenless. This reduced roller seating angle β′ promotes faster separationwhen the roller leaves the sprocket and enters the span. This reducedangle β′ also allows for the roller in a worn chain to ride up the coastflank surface to a less severe angle as the roller moves around thesprocket in the wrap.

It is contemplated that the above-described asymmetrical tooth profilefeatures can be altered without substantially deviating from the chainand sprocket meshing kinematics that produce the noise reductionadvantages of the present invention. For example, the engagingasymmetrical flank profile could be approximated by an involute form,and the disengaging asymmetrical flank profile could be approximated bya different involute form. Slight changes to the asymmetrical toothprofiles may be made for manufacturing and/or quality control reasons,or simply to improve part dimensioning. These changes are within thescope of the invention as disclosed herein.

In a further embodiment, the engaging flank inclined root surface 146(FIG. 9) may be replaced with a coast flank inclined root surface 164 asshown in FIG. 18. The coast flank inclined root surface 164 providestooth space clearance (TSC) in the same manner as described above withregard to the inclined root surface 146. In addition, the disengagingflank inclined root surface 164 beneficially moves the roller to apreferred radially outward position as the chain wears.

Alternatively, the coast flank inclined root surface 164 may be includedwith the engaging flank inclined root surface 146 as shown in FIG. 19.The engaging flank and coast flank inclined root surfaces 146, 164cooperate to provide tooth space clearance (TSC) in the same manner aspreviously described.

Referring now to FIG. 20, any one of the above-described asymmetricaltooth profile embodiments of FIGS. 9, 15, 18, and 19 may be incorporatedinto a random-engagement roller chain sprocket 300. The sprocket 300 isshown as an 18-tooth sprocket. However, the sprocket 300 may have moreor less teeth, as desired. The sprocket 300 includes a first group ofarbitrarily positioned sprocket teeth 302 each having a profile whichincorporates the flank flat 144 shown in FIGS. 9, 15, 18 and 19.Further, the sprocket teeth 302 can incorporate none, one or bothinclined root surfaces 146, 164 as shown in FIGS. 9, 15, 18 and 19. Theremaining sprocket teeth 304 (sprocket teeth 1, 3, 4, 9, 13, 14, and 16)are randomly positioned around the sprocket and incorporate a toothprofile different from that of the first group of sprocket teeth 302. Asdescribed further below, the first and second groups of sprocket teeth302, 304 cooperate to reduce chain drive system noise levels below anoise level which either tooth profile used alone would produce.

FIG. 21 illustrates an exemplary tooth profile for one of the sprocketteeth 304. An engaging flank 306 and a coast or disengaging flank 308 ofan adjacent tooth cooperate to define a tooth space 310 having a rootsurface 311. The tooth space 310 receives an engaging roller 314 (shownin phantom). The engaging roller 314 has a roller diameter D₁ and isshown fully seated with single point (line) contact within the toothspace 310. As best seen in FIG. 21a, the engaging roller 314 does notcontact the engaging flank 306 at the onset of meshing, moving insteaddirectly from the span to full-mesh root contact on an inclined rootsurface 316 at a contact point C′ located radially outward of contactpoint C in a direction toward the engaging flank 306. Contact point C′is a roller/tooth contact line that extends axially along each sprockettooth surface (i.e., in a direction orthogonal to the plane of thedrawings). Thus, a clearance 321 is defined between the roller 314 andthe engaging flank 306.

As shown in FIGS. 21 and 21a, the first or engaging root radius R_(i) istangent to inclined root surface 316 at line 319, and also tangent toR_(f) as defined by angle β. Angle β has no functional requirementsspecific to the onset of roller meshing since roller/flank contact willnot take place with tooth profile 304. It should be noted that R_(i) canbe equal to the ISO-606 root radius for tooth profile 304.

The length of the inclined root surface from point C to its radiallyouter end at line 319 is determined by the amount of flank offsetrequired to ensure that the roller will not have engaging flank contactfor any expected chain pitch elongation (wear) throughout its designlife. In the preferred embodiment, the flank offset 323 is in the rangeof about 0.025 to about 0.13 mm. The flank offset 323 is an extension ofinclined root surface 316 which provides tooth space clearance (TSC) inthe same manner described above with regard to inclined root surface146.

As previously mentioned, tooth space clearance compensates for chainpitch elongation or chain wear by accommodating a specified degree ofchain pitch elongation. In other words, the tooth space clearance TSCenables rollers of a worn chain to be maintained in hard contact withthe inclined root surface of the sprocket teeth. In addition, theinclined root surface 316 facilitates reducing the radial reaction forcethereby reducing the roller radial impact noise contribution to theoverall noise level.

The inclined root surface 316 may be inclined at any angle φ necessaryto satisfy a specific chain drive geometry and chain pitch elongation.The inclined root surface angle φ is measured from a line 320 passingthrough the center of roller 314 and the sprocket center to a secondline 322 which also passes through the center of roller 314 and throughcontact point C. The inclined root surface 316 is normal to the line322, and the inclined root surface extends radially inward to line 318where it is tangent to R_(i)′. In the embodiment being described, theinclined root surface angle φ is preferably in the range of about 20° toabout 35°.

FIG. 22 shows another embodiment of the tooth profile 304 where no toothspace clearance TSC is provided. That is, the flat surface portion ofthe inclined root surface from line 318 to line 322 is not present.Thus, the profile of the tooth 304 from point C to the outside or tipdiameter at the disengaging side of the tooth space is substantiallyidentical to the tooth profile shown in FIG. 15. The remaining flatsurface portion 323 of the inclined root surface 316 from line 322 toline 319 functions only to provide the engaging flank offset asdescribed above.

It should be appreciated that the portion of the engaging flank inclinedroot surface from line 318 to line 322 may be replaced with a coastflank inclined root surface 164 as in FIG. 18. That is, the toothprofile 304 may be substantially identical to the tooth profile forsprocket 112 shown in FIG. 18 from contact point C to the outer diameterof the coast flank 138. The coast flank inclined root surface 164provides tooth space clearance (TSC) in the same manner as the inclinedroot surface 316. In addition, the coast flank inclined root surfacebeneficially moves the roller to a preferred radially outward positionas the chain wears. Alternatively, the coast flank inclined root surface164 may be included with the engaging flank inclined root surface 316 inthe same manner as shown in FIG. 19 (where the inclined surface 164 isincluded with the inclined surface 146). Thus, the tooth profile 304 mayalso be substantially identical to the tooth profile for sprocket 112shown in FIG. 19 from contact point C to the outer diameter of the coastflank 138.

Pitch mismatch is inherent in a chain/sprocket interface except at onecondition—the theoretical condition which is defined as a chain at itsshortest pitch (shortest being theoretical pitch) and a maximum materialsprocket. This theoretical condition therefore defines one limit (zero,or no pitch mismatch) of the tolerance range of the pitch mismatchrelationship of chain and sprocket. The other limit is defined when alongest “as built” chain is used with a sprocket at minimum materialconditions—or in other words, a sprocket having a minimum profile. Thislimit produces the greatest amount of pitch mismatch. The pitch mismatchrange is therefore determined by the part feature tolerances.

Additional pitch mismatch may be introduced to facilitate a greater timedelay, or “staged” meshing, between the initial tangential contact atpoint A and the full seated contact at points B and C for tooth profile302. It should be appreciated that staged contact for profile 302 isenhanced due to the flank flat 144 which causes initial contact to occurhigher up on the engaging flank. This will result in reduced meshfrequency noise levels because the point and rhythm of the initialroller-to-sprocket contacts are altered for each tooth profile 302, 304since profile 304 does not have a tangential contact.

The sprocket chordal pitch is necessarily shorter than the chain pitchto facilitate the “staged” roller-tooth contact. In addition, chordalpitch reduction also provides roller-to-flank clearance as the rollerexits the sprocket wrap back into the span.

FIG. 29 partially illustrates an asymmetric sprocket 112 including aplurality of teeth 132 of the same profile. For convenience and ease ofunderstanding the feature of added chordal pitch reduction (CPR),successive teeth 132 are identified using reference numerals including asuffix “a,” “b,” etc., i.e., 132 a, 132 b, 132 c. The sprocket 112 isintended to rotate in a clockwise direction 11 and, accordingly, eachtooth 132 includes an engaging flank 134 and a disengaging flank 138. Atooth space 140 is defined between the engaging flank 134 of each toothand the disengaging flank of the next or successive tooth 132. A rollerchain comprising rollers 128,130 and links (the links are not shown) isengaged with the sprocket 112, with the rollers 128,130 fully seated intheir respective tooth spaces 140.

As described above, the roller chain includes a chain pitch P_(c) whennew or “as built” that is defined by the distance from center-to-centerof successive rollers 128, 130. Sprocket chordal pitch is defined hereinas the straight-line distance between the centers of successive rollersif both rollers are seated in full-mesh driving contact with thesprocket when the sprocket is in a new or “as-manufactured” condition.Thus, for the sprocket 112, illustrated in FIG. 29, the sprocket chordalpitch P_(s) is the straight-line distance between the centers of therollers 128, 130 when both rollers are engaged at points B and C ontheir respective engaging flanks 134. In the example illustrated in FIG.29, the sprocket 112 includes a chordal pitch P_(s) that isapproximately 0.1% reduced relative to the chain pitch P_(c) due tomanufacturing tolerances. Thus, as illustrated in FIG. 29, the roller128 is seated in two-point driving contact with the engaging flank 134of the tooth 132 b, i.e., the roller is engaged with points B and C onthe engaging flank 134 of the tooth 132 b. The next-meshing roller 130has just engaged the engaging flank 134 of the tooth 132 a at an initialcontact point A. However, as illustrated, point A is very nearlycoincident with point B because the sprocket chordal pitch P_(s) isnominally equal to the chain link pitch P_(c). Thus, the roller 130essentially seats in two-point driving contact with points B and C ofthe engaging flank 134 of the tooth 132 a, even though the roller 130has just engaged the sprocket 112. Accordingly, those of ordinary skillin the art will recognize that staged-impact, i.e., an extension of themeshing time interval, is not present to any meaningful degree in FIG.29. In other words, the roller 130 undesirably seats in two-pointdriving contact at points B and C almost immediately upon engaging thesprocket 112. In this case, the chordal action (chordal rise and fall)as indicated at CA and defined above is larger than desired.

FIG. 30 partially illustrates a sprocket 112′ that is identical to thesprocket 112 except that it is defined to include added chordal pitchreduction (CPR) when in a new or “as-manufactured” condition, i.e., thesprocket chordal pitch P_(s)′ is purposefully reduced as compared to theas-built chain pitch P_(c) a select amount referred to herein as “addedchordal pitch reduction.” As described below, this purposefully addedchordal pitch reduction is in addition to the inherent chordal pitchreduction that exists owing to manufacturing tolerances.

Added chordal pitch reduction in accordance with the present inventionand as shown in FIG. 30 is highly desired in that it extends the meshingtime interval, i.e., it enhances staged impact of a next-meshing roller130 with the sprocket 112′. This is readily apparent by comparing FIGS.29 and 30. In FIG. 30, it can be seen that the roller 128 is fullyengaged and seated in two-point driving contact with the engaging flank134 of the tooth 132 b, i.e., the roller 128 is in contact with bothpoints B and C. The next-meshing roller 130 has just engaged thesprocket 112′ and is not yet fully seated in two-point driving contact.Instead, the next-meshing roller 130 makes initial contact with theengaging flank 134 at point A and, owing to the added chordal pitchreduction in accordance with the present invention, the point A isspaced radially outward from point B. Therefore, the roller 130 willseat in driving contact with points B and C only upon further rotation11 of the sprocket 112′. As described above, this delay between initialcontact of the roller 130 at point A and subsequent engagement of theroller with points B and C is a highly desired staged impact orextension of the meshing time interval.

This movement of the initial contact point A radially outwardly for thesprocket 112′ as compared to the sprocket 112 is readily apparent bycomparing FIGS. 29 and 30. Also, further comparison of FIGS. 29 and 30indicates that, use of the sprocket 112′, including added chordal pitchreduction, also reduces the chordal rise and fall action CA′ as comparedto the chordal action CA of the sprocket 112.

The preferred amount of added chordal pitch reduction is best understoodwith reference now to FIG. 31. There, the sprocket 112 is illustratedtogether with the rollers 128 and 130, both of which are fully seated intwo-point driving contact with their respective engaging flanks 134 atpoints B and C. As noted above, the sprocket 112 has no added chordalpitch reduction. The line P_(s) 1 indicates a maximum chordal pitchbased upon a standard maximum “over-pin” tolerance. The line P_(s) 2indicates a minimum chordal pitch based upon a standard minimum over-pintolerance. In particular, the line P_(s) 2 indicates a chordal pitchreduction of approximately 0.2% as compared to the maximum chordal pitchP_(s) 1 owing to manufacturing tolerances. Thus, those of ordinary skillin the art will recognize that the sprocket chordal pitches P_(s)1,P_(s) 2 represent the maximum and minimum sprocket chordal pitchdimensions for a sprocket 112 with no added chordal pitch reduction. Inother words, a sprocket with no added chordal pitch reduction willexhibit, at most, chordal pitch reduction of approximately 0.2% due tomanufacturing tolerances as indicated by the line P_(s) 2.

With continuing reference to FIG. 31, the line P_(s) 3 indicates themaximum sprocket chordal pitch based upon a maximum over-pin tolerancefor a sprocket having 1% added chordal pitch reduction. The line P_(s) 4indicates the minimum chordal pitch based upon a minimum over-pintolerance for a sprocket with 1% added chordal pitch reduction. In otherwords, the reduced sprocket chordal pitch represented by the line P_(s)3 indicates a 1% reduction of the maximum possible sprocket chordalpitch P_(s) 1 for a sprocket 112 with no added chordal pitch reduction.Likewise, the reduced sprocket chordal pitch represented by the lineP_(s) 4 indicates a 1% reduction of the minimum chordal pitch P_(s) 2for a sprocket 112 with no added chordal pitch reduction.

It is most preferred that a sprocket 112′ formed in accordance with thepresent invention include no more than 1% added chordal pitch reductionrelative to the maximum chordal pitch P_(s) 1 for a sprocket with noadded chordal pitch reduction (which is equal to the as-built chain linkpitch P_(c)). The sprocket 112 illustrated in FIG. 29 has no addedchordal pitch reduction but, due to manufacturing tolerances, exhibitsnominal chordal pitch reduction of about 0.1%, i.e., the chordal pitchof the sprocket 112 would fall midway between the chordal pitch P_(s) 1and P_(s) 2 shown in FIG. 31. The sprocket 112′ illustrated in FIG. 30and formed in accordance with the present invention incorporates addedchordal pitch reduction of 0.5% relative to the sprocket 112 illustratedin FIG. 29.

The sprocket 112′ incorporating added chordal pitch reduction inaccordance with the present invention can be an asymmetric sprocketincluding nominally identical asymmetric tooth profiles. The teeth on asprocket formed in accordance with the present invention can be formedwith any of the asymmetric profiles described herein. Alternatively, thesprocket 112′ can be a random asymmetric sprocket 300 as describedherein that includes multiple different asymmetric tooth profiles on asingle sprocket. Also, a sprocket featuring added chordal pitchreduction in accordance with the present invention can be a drivesprocket or a driven sprocket, and a roller chain drive system formed inaccordance with the present invention can include a drive sprocketand/or a driven sprocket incorporating added chordal pitch reduction inaccordance with the present invention.

Finally, it should be noted that, although the asymmetric tooth profilesillustrated herein are conformed to result in the rollers 128, 130seating in two-point driving contact therewith, other arrangements arecontemplated. In particular, a sprocket incorporating one-point seatingasymmetric tooth profiles can also incorporate added chordal pitchreduction in accordance with the present invention. For example,commonly assigned U.S. Pat. No. 5,876,295 discloses additionalasymmetric tooth profiles and sprockets that can include chordal pitchreduction in accordance with the present invention. Accordingly, thedisclosure of U.S. Pat. No. 5,876,295 is hereby expressly incorporatedby reference herein. Other asymmetric tooth profiles that may be used ona sprocket formed in accordance with the present invention are describedin commonly assigned U.S. Pat. Nos. 5,921,878 and 5,993,344, and thedisclosures of both of these patents are hereby expressly incorporatedby reference herein.

The staged roller contact for tooth profile 302 may be further assistedby providing a sprocket tooth pressure angle γ that is substantiallyless than the ISO-606 standard. Pressure angles γ equal to or very closeto zero (0), or even negative pressure angles, are contemplated.

FIG. 23 illustrates the tooth profile 304 overlaid with the toothprofile 302 (phantom). An engaging roller is shown at the onset ofinitial tangential contact at point A along the engaging flank of thetooth profile 302. The engaging roller will maintain contact with theengaging flank until it is at full mesh and seated at points B and C aspreviously described with reference to FIG. 9. The tooth profile 304 isoverlaid on the tooth profile 302 to show that an engaging roller onlyhas radial contact on the inclined root surface of the tooth profile 304(see FIGS. 21a and 22) and does not have tangential contact with thetooth profile 304.

The pressure angle γ for tooth profile 304 has no functional purposeduring the onset of roller meshing since the roller does not contact theengaging flank. The pressure angle γ₃₀₂ for the tooth profile 302 isshown as a negative value. Thus γ_(min) may be a small negative value,and γ_(max) may be a positive value equal to some value less than theISO-606 minimum pressure angle γ. As a result, initialroller-to-sprocket contact for tooth profile 302 of sprocket 300 (FIG.20) occurs at point A followed by full engagement contact at points Band C. The sprocket 300 may, or may not incorporate additional chordalpitch reduction, and may, or may not incorporate tooth space clearance(TSC), as described above.

FIG. 24 shows the engagement path of a roller 342 from initial contactat point A (phantom) to fully-seated, two-point contact (solid) in asprocket tooth 302, and an engagement path of the roller 314 engaging anadjacent sprocket tooth 304 of the random engagement sprocket 300. Atthe onset of meshing for roller 314, a small portion of the chain loadtransfer takes place with tooth 304 picking up a share of the loading.However, tooth 302 continues to carry a larger portion of the chainloading until an engaging roller meshes with another tooth 302 with itsattendant flank contact. The reference l in FIG. 24 indicates the amountof “staging” for the tooth 302 from the initial contact at point A tothe full mesh contact at points B and C.

FIGS. 25 and 26 illustrate the meshing delay between the tooth profiles302, 304. In particular, as shown in FIG. 25, the sprocket 300 has afurther roller 344 fully-seated in two-point contact with a sprockettooth incorporating the tooth profile 302. The roller 342 is shown atthe instant of initial tangential contact at point A of a secondsprocket tooth also incorporating the tooth profile 302. The roller 314is the next roller in the span and will mesh with a sprocket toothincorporating the tooth profile 304. The sprocket 300 must rotatethrough an angle τ for roller 342 to move from its initial contactposition at point A to full mesh, seated in two-point contact with thetooth profile 302 at a 12 o'clock position.

With reference to FIG. 26, the sprocket 300 of FIG. 25 is shown rotatedin a clockwise direction until roller 314 is at the onset of meshingwith the tooth profile 304. The sprocket 300 must now rotate through asmaller angle K to have roller 314 seated at the 12 o'clock position.Thus, the sprocket 300 must rotate through an additional angle T-K foran engaging roller to be fully seated in the tooth profile 302.

Referring again to FIG. 20, the two sets of tooth profiles 302, 304 arearranged in a random pattern in order to modify the meshing impactfrequency by altering the point and rhythm of initial roller-to-sprocketcontact. However, the two sets of tooth profiles 302, 304 could bearranged in many different random patterns. Further, it is alsocontemplated that the two sets of tooth profiles 302, 304 could bearranged in many regular patterns that would work equally as well. Inall cases, the arrangement of two sets of different tooth profiles on asprocket provides a means for breaking up the mesh frequency impactnoise normally associated with and induced by a full complement ofsubstantially identically shaped sprocket teeth. The mesh frequencynoise reduction is achieved by altering the point and rhythm of initialroller-to-sprocket contact.

The crankshaft sprocket, generally the smallest sprocket in the chaindrive, is usually the major noise contributor. The typically largerdriven camshaft sprocket, however, will also contribute to the generatednoise levels, but generally to a lesser extent than the crankshaftsprocket. However, the driven sprocket, particularly if it is nearly thesame size or smaller than the driving sprocket, may be the prime noisegenerator, as in the case with balance shaft sprockets and pumpsprockets. Thus, the features of the present invention may also be usedadvantageously with camshaft or other driven sprockets as well.

It should be appreciated that the tooth profile features of FIGS. 20-26can be altered slightly without substantially deviating from the chainand sprocket meshing kinematics that produce the noise reductionadvantages of the present invention. For example, the engagingasymmetrical flank profile could be approximated by an involute form,and the disengaging asymmetrical flank profile could be approximated bya different involute form. Slight changes to the profile may be done formanufacturing and/or quality control reasons—or simply to improve partdimensioning.

The invention has been described with reference to the preferredembodiments. Obviously, modifications will occur to others upon areading and understanding of this specification and this invention isintended to include same insofar as they come within the scope of theappended claims or the equivalents thereof.

Having thus described the preferred embodiments, the invention is nowclaimed to be:
 1. A unidirectional roller chain drive system comprising:an as-manufactured drive sprocket including a plurality of teeth; anas-manufactured driven sprocket including a plurality of teeth; and, anas-built roller chain engaged with said drive sprocket and said drivensprocket, said roller chain defining a chain link pitch (P_(c)), whereinat least one of said drive sprocket and said driven sprocket defines asprocket chordal pitch (P_(s)′) that is less than said chain link pitch(P_(c)) by more than 0.2% of said chain link pitch (P_(c)).
 2. Theunidirectional roller chain drive system as set forth in claim 1,wherein said at least one of said drive sprocket and said drivensprocket comprises a plurality of teeth each having an asymmetricprofile.
 3. The unidirectional roller chain drive system as set forth inclaim 2, wherein said at least one of said drive sprocket and saiddriven sprocket comprises first and second different asymmetric toothprofiles.
 4. The unidirectional roller chain drive system as set forthin claim 3, wherein said first and second different asymmetric toothprofiles are arranged arbitrarily on said at least one sprocket.
 5. Theunidirectional roller chain drive system as set forth in claim 2,wherein each of said plurality of teeth includes an engaging flankconformed to engage rollers of said roller chain in one-point drivingcontact in a full mesh position.
 6. The unidirectional roller chaindrive system as set forth in claim 2, wherein each of said plurality ofteeth includes an engaging flank conformed to engage rollers of saidroller chain in two-point driving contact in a full mesh position. 7.The unidirectional roller chain drive system as set forth in claim 2,wherein said teeth defines tooth spaces therebetween, said tooth spacesdefining an engaging flank pressure angle in the range of about −2° toabout +50°.
 8. The unidirectional roller chain drive system as set forthin claim 1, wherein said sprocket chordal pitch P_(s)′ is less than saidchain link pitch P_(c) by more than 0.2% but less than or equal to 1%.9. An as-manufactured sprocket adapted for use in conjunction with anassociated as-built roller chain comprising a plurality of rollers andthat defines a chain pitch P_(c), said sprocket comprising: a pluralityof sprocket teeth projecting outwardly therefrom and defining aplurality of tooth spaces located respectively between successive teeth,each of said tooth spaces adapted to receive a roller of said associatedroller chain, said sprocket defining a sprocket chordal pitch P_(s)′that is reduced relative to said chain pitch P_(c) of said associatedroller chain by more than 0.2% of said chain pitch P_(c).
 10. Thesprocket as set forth in claim 9, wherein said tooth spaces areasymmetric tooth spaces.
 11. The sprocket as set forth in claim 10,wherein said tooth spaces are defined with an engaging flank pressureangle in the range of about −2° to about +5°.
 12. The sprocket as setforth in claim 9, wherein said plurality of sprocket teeth areasymmetric teeth and each include an engaging flank and a disengagingflank, said engaging flank of each of said plurality of sprocket teethadapted to engage the associated rollers of said associated roller chainin one-point driving contact.
 13. The sprocket as set forth in claim 9,wherein said plurality of sprocket teeth are asymmetric teeth eachincluding an engaging flank and a disengaging flank, said engaging flankof each tooth dimensioned and conformed relative to the associatedrollers of the associated roller chain so that said engaging flanksengages the associated rollers in two-point driving contact at rollerseating points (B,C) when the associated roller is in full mesh in oneof said tooth spaces.
 14. The sprocket as set forth in claim 13, whereinsaid reduced sprocket chordal pitch P_(s)′ causes the associated rollersof the associated roller chain make initial contact with respectiveengaging flanks of said teeth at a point (A) spaced radially outwardlyfrom said points (B,C) whereby the time between the associated rollercontacting the point (A) and seating at points (B,C) is lengthened. 15.The sprocket as set forth in claim 9, wherein said plurality of sprocketteeth are asymmetric teeth of at least two different profiles arrangedirregularly on said sprocket.
 16. The sprocket as set forth in claim 15,wherein said sprocket chordal pitch P_(s)′ is reduced relative to saidchain pitch P_(c) of said associated roller chain by more than 0.2% ofsaid chain pitch P_(c) and less than or equal to 1% of said chain pitchP_(c).
 17. A method of extending the meshing interval for an as-builtroller chain and an as-manufactured sprocket, said method comprising:defining said sprocket to have a sprocket chordal pitch P_(s)′ that isreduced relative to a chain link pitch P_(c) of the roller chain by morethan about 0.2% of the chain link pitch P_(c).
 18. The method as setforth in claim 17, wherein said sprocket chordal pitch P_(s)′ is reducedrelative to the chain link pitch P_(c) by more than about 0.2% of thechain link pitch P_(c) and less than or equal about 1% of the chain linkpitch P_(c).
 19. The method as set forth in claim 18, furthercomprising: defining said sprocket to have tooth spaces betweenconsecutive teeth that are defined with an engaging flank pressure anglein the range of about −2° to about +5°.